1. Field of the Invention
The present invention relates generally to hard disk drives, and more particularly to a method of making a balance-compensated (BC) rotary actuator for use in a rotatable BC head stack assembly (HSA) portion in a disk drive based upon track-follow characteristics of a rotatable test HSA portion.
2. Description of the Prior Art
A representative disk drive incorporating the structures referred to herein is taught, for example, in U.S. Pat. No. 5,953,183 to Butler et al., entitled xe2x80x9cHEAD STACK ASSEMBLY FOR A MAGNETIC DISK DRIVE WITH A PASS-THROUGH FLEX CIRCUIT CABLE,xe2x80x9d and is incorporated herein by reference.
A disk drive includes a disk having a track, a disk controller for generating a servo control signal, and a head stack assembly. The head stack assembly includes a rotary actuator and a flex cable assembly. The rotary actuator includes an actuator body portion, a pivot axis extending through the actuator body portion, and a head extending from the actuator body portion. The rotary actuator is responsive to the servo control signal for positioning the head over the track.
The flex cable assembly includes a flex circuit cable, a flex clamp and electrical components (e.g., an integrated circuit containing a pre-amplifier). The flex cable is interposed between a printed circuit board assembly and the integrated circuit. The head stack assembly includes a rotatable head stack assembly (HSA) portion which includes a rotatable flex cable portion for receiving the servo control signal and the rotary actuator. As such, the rotatable HSA portion includes those subcomponents of the head stack assembly which are configured to rotate about the pivot axis.
It is known that gravitational effects on a mass of the attached flex cable portion and other components of the head stack assembly produce a torque on the rotary actuator about an axis which extends perpendicularly from the pivot axis. Such a torque is undesirable as the rotary actuator becomes susceptible to an external acceleration torque being applied to the rotary actuator about the pivot axis when the disk drive is exposed to an external linear acceleration. The external linear acceleration may be due to vibration or shock for example. The resulting external acceleration torque results in positional errors of the heads relative to the desired tracks (off-track errors) being introduced into the system during operation.
Off-track errors are particularly undesirable for a number or reasons. In particular, such errors directly impact the overall seek time of the disk drive because the settling time will increase. Further, the data transfer rate will decease due to the off-track errors. Moreover, as the tracks per inch (TPI) specification of disk drives increase, the system sensitivity of such off-track errors likewise increases.
There have been attempts to mitigate against gravitational effects of the mass of the flex cable and other components which produce a torque applied to the rotary actuator. In this regard, a known methodology includes estimation of which portion of the flex cable is associated with a gravitational related torque applied to the rotary actuator related to the mass of the flex cable, in addition to the mass other components of the head stack assembly. For example, half of the flex cable may be a rough estimate. The prior art method would call for cutting the flex cable at the estimated location and removing the rotary actuator with the severed portion of the flex cable. The center of gravity of the rotary actuator with the severed portion of the flex cable would then be determined by mechanical means. Using such center of gravity information, the rotary actuator design would then be modified so as to attempt to locate the center of gravity of the rotary actuator with the severed portion of the flex cable at the pivot axis. Such prior art rotary actuator designs have proven to include a high degree of off-track errors associated with external linear acceleration acting on the disk drive. Accordingly, there is a need in the art for a method of making an improved rotary actuator as incorporated in a disk drive.
An aspect of the invention can be regarded a method of making a balance-compensated (BC) rotary actuator for use in a rotatable BC head stack assembly (HSA) portion in a disk drive based upon track-follow characteristics of a rotatable test HSA portion is provided by the present invention. The test HSA portion includes a test rotary actuator having a test actuator weight distribution specification. The test rotary actuator has an actuator body portion, a test pivot axis extending through the actuator body portion and a head connected to the actuator body portion. The test rotary actuator is controlled to rotate about the test pivot axis for positioning the head over a selected disk track. The test rotary actuator has a test rotary actuator center-of-gravity torque vector associated therewith about the test pivot axis. The method provides for vibrating the disk drive at a vibration frequency in order to vibrate the test HSA portion. Position error information is read using the head while performing a track-follow operation during the vibrating of the test HSA portion. Head off-track error due to the vibration of the disk drive is derived based upon the read position error information representative of positioning of the head relative to the selected disk track. A resultant torque vector is derived based upon the head off-track error. The resultant torque vector has a magnitude equal to a mass of the test HSA portion times a distance between the test pivot axis and a test installed HSA portion center-of-gravity. The test installed HSA portion center-of-gravity takes into consideration forces acting upon the test rotary actuator due to installation with the test HSA portion. The resultant torque vector has a direction extending perpendicular from the test pivot axis towards the test installed HSA portion center-of-gravity. A balancing torque vector is derived having a magnitude equal to and a direction opposite of the resultant torque vector. The balancing torque vector is incorporated into the test actuator weight distribution specification to produce a BC actuator weight distribution specification. The BC actuator weight distribution specification is used to manufacture the BC rotary actuator having a BC pivot axis. The BC HSA portion has a BC HSA installation center-of-gravity at the BC pivot axis for mitigating external acceleration applied to the BC rotary actuator about the BC pivot axis due to external linear acceleration experienced by the disk drive during a track-follow operation.